1. Field of the Invention
This invention relates to a process for solvent dewaxing waxy petroleum oils by an improved incremental dilution method. In one of its more specific aspects, the process comprises cooling the lubricating oil base stock, with or without predilution with solvent, with vigorous agitation during the early stages of cooling, i.e. to a temperature below its cloud point. The cooling then may be continued with incremental solvent addition in a conventional manner to the desired final filtration temperature. The present process results in the formation of wax crystals which have improved filter rates, and in improved dewaxed oils yields as compared with conventional incremental dilution methods. The process may be carried out in plants having existing scraped wall chillers modified as described hereinafter, with a minimum of investment, as the process requires no special crystallizer.
2. Description of the Prior Art
The incremental dilution method of solvent dewaxing of waxy petroleum lubricating oil base stock is a common industrial technique. In this method, a suitable solvent is added to the oil base stock at several points during the chilling process. The waxy oil may be chilled at the rate of 1 to 5.degree. F. per minute. Suitable solvents, for example, propane, mixtures of methyl ethyl ketone (MEK) and toluene, mixtures of dichloromethane and dichloroethylene, and the like, are added to the base stock and to the resulting petroleum oil-solvent mixture as cooling progresses. The process is usually carried out in double walled tubular heat exchangers provided with mechanical scrapers in the inner conduit and means for supplying cooling medium to the annulus.
In the conventional incremental dilution method, the temperature of the solvent preferably is at or slightly above that of the oil or the oil-solvent mix at the point of addition. If the solvent is at a lower temperature, undesirable shock chilling occurs resulting in the formation of numerous small wax crystals, and if it is too high, it adds unnecessary additional heat load to the scraped surface exchangers. Capacity of a given solvent dewaxing unit often is limited by the heat exchange surface in the chillers.
It is known from U.S. Pat. No. 3,642,609, and related patents, that intensive agitation of a waxy petroleum oil stock accompanied by dilution chilling and the stimultaneous incremental addition of cold solvent to the system gives high rates of filtration of the resulting wax crystals from the oil with good yields of dewaxed oil. Special equipment is required for the cooling and dilution operations.